Thermally compensated laser discharge structure

ABSTRACT

A gas laser including a quartz insulator tube enclosing a stack of graphite discs having a central aperture forming a laser discharge path wherein the individual discs are spatially separated and electrically insulated from one another by quartz rods inserted into spacer holes disposed about the central aperture, the depth of the spacer holes in the discs intermediate the end discs being constructed and arranged in a manner to compensate for thermally induced variations in the length of the stack.

United States Patent 3,50l,7l4 3/1970 Myers et al.

lnventors Karl J. Knudsen East Rockaway: Robert J. Gartner. C arlePlace, both of, N.Y.

Appl. No. 778,529

Filed Nov. 25, 1968 Patented Aug. 10, 1971 Assignee Sperry RandCorporation THERMALLY COMPENSATED LASER DISCHARGE STRUCTURE 6 Claims, 4Drawing Figs.

US. Cl 330/43, 313/204, 33 l/94.5 Int. Cl. "01s 3/05 Field of Search330/43; 331/945; 313/204 References Cited UNITED STATES PATENTS oTH ERREFERENCES Knudsen et al., Argon lon Lasers, Sperry Engr. Rev, v01.19,No. 1, 1966, pp. 27 31 Hernquist et aL, Construction of Long LifeArgon Lasers, IEEE J. Quantum Electronics, Vol. QE-3, No. 2, Feb. i967,pp. 66- 72 Primary ExaminerRodney D. Bennett, Jr. AssistantExaminer-Daniel C. Kaufman Attorney-S. C. Yeaton ABSTRACT: A gas laserincluding a quartz insulator tube enclosing a stack of graphite discshaving a central aperture forming a laser discharge path wherein theindividual discs are spatially separated and electrically insulated fromone another by quartz rods inserted into spacer holes disposed about thecentral aperture, the depth of the spacer holes in the discsintermediate'the end discs being constructed and arranged in a manner tocompensate for thermally induced variations in the length of the stack.

PATENTEDAUBIOIB?! 3,599.10?

SHEET 1 UF 2 FIG.1.

I/VVE/VTORS KARL J. K/VUDSE/V ROBERT GART/VER THERMALLY COMPENSATEDLASER DISCHARGE STRUCTURE BACKGROUND OF THE INVENTION The presentinvention relates to lasers and more particularly to means for providingthermal compensation in high-power gas laser devices of the type inwhich the discharge bore is formed by a central aperture in a stack ofrefractory members.

Early state of the art gas lasers, as exemplified by the helium-neonsystem, generally operated in the infrared or long wavelength end of thevisible spectrum and were inherently low-power devices capable ofproviding only milliwatts of power output in sustained continuous waveoperation. These devices produced lasing action by means of energytransitions existing between excited atoms in the gas. In the interestof achieving higher power output and extending the operative frequencyrange, the lasing properties of various other gases were investigated.This research led to the development of the ion laser in which lasingaction is produced from the transitions between excited states of theions in a gaseous discharge. To date, these devices have operatedsuccessfully with the noble gases, particularly argon, and have producedlight beams extending through the short-wavelength end of the visiblespectrum into the ultraviolet region at power levels on the order of lwatt or more in sustained continuous wave operation. To obtain the highpower output available with ion lasers, however, a high current must beestablished in the discharge path and therefore intense heat developswithin the tube. The extent of the heating problem can be readilyappreciated when it is realized that the temperature in the dischargetube rises at least 1,000 C. and perhaps as much as a few thousanddegrees when lasing operation commences under high discharge currentconditions. Since the interior diameter of the discharge tube istypically a few millimeters, theamount of tube wall surface area throughwhich the heat can be dissipated is very small. Hence, ordinary glass,quartz and similar ceramic materials are not suitable for high-powerdevices. These materials have not only comparatively low safe operatingtemperatures but also low thermal conductivity which inhibits heatradiation into the ambient environment. Consequently, cooling by meansof a water jacket surrounding the exterior wall of the tube does notalleviate the problem, since the water merely cools the exterior surfaceof the tube causing a large temperature difference to exist across thetube wall.

In addition to the heat problem, the high current is accompanied byintense ion bombardment of the tube wall. In the case of quartz, forexample, the ion bombardment releases cathode poisoning gases in thetube and causes erosion of the tube wall leaving residues thereon whichare subsequently heated by the hot gas resulting in localized arcing andcatastrophic puncturing. Ion bombardment also causes gas cleanup, thatis, loss of gas through the tube wall, at a rate of l to 2 percent perhour which is intolerable in view of the low operating pressure andnarrow range of pressures required for optimum lasing action.

As a result of the difficulties experienced with tubes constructed ofthe aforementioned materials, a fairly recent innovation in the ionlaser art disclosed a quartz insulator tube enclosing a stack ofrefractory, high thermal conductivity discs having a central boreforming the laser discharge path, the individual refractory membersbeing spatially separated from one another by intervening insulatingwashers constructed of a material such as fused quartz. In thisinstance, the intense heat produced by the high current discharge isconfined to the small central bore in the discs from which it is quicklyradiated through the substantially larger surface area of the quartzvacuum envelope enclosing the discs. Quartz is a suitable insulator tubematerial for devices of this type because the insulator tube diameter istypically 1 inch or larger and thus provides a substantially increasedtube wall area through which the heat is radiated. Other materials suchas molybdenum and tantalum have been investigated for constructing therefractory discs but graphite is preferred because of its low cost, easymachinability, high operating temperature, relatively high thermalconductivity and resistance to corrosion by ion bombardment. Graphitehas the disadvantage, however, of having a comparatively large thermalcoefficient of expansion in a direction parallel to the central bore. Infact, the high temperature produced in the central bore of the discsduring lasing operation is sufficient to cause approximately 1centimeter of expansion in l-meter-long stack. Consequently, in a casewhere the stack is held rigidly in place within the quartz envelopeunder nonoperating conditions, the expansion occurring during operationplaces ex treme tension on the quartz tube causing it to crack orshatter. I-Ieretofore, this problem has been solved by threading thegraphite discs onto sapphire rods which are slightly longer than thestack under nonoperating conditions and fixedly secured to the insulatortube. The rods prevent the discs from canting to assure that alignmentof the discharge bore is maintained and also enable the graphite stackto expand longitudinally by a precise amount so that the stack forms arigid structure under operating conditions without applying tension tothe insulator tube. This design is highly susceptible to shock andvibration when the laser is not in use inasmuch as the stack is able tomove to and fro along the sapphire rods thereby severely impairing tubelife and reliability.

SUMMARY OF THE INVENTION The present invention overcomes theaforementioned disadvantages caused by longitudinal expansion of thedischarge tube forming stack by the provision of unique means forelectrically insulating and spatially separating the individualrefractory members of the stack. In place of the quartz washers used asinsulating spacers in the prior art, in the present invention aplurality of fused quartz insulating rods are inserted in spacer holesdisposed about the central bore of the refractory members and extendingonly partially therethrough to provide spatial separation and electricalinsulation therebetween. More specifically, the spacer holes in therefractory members intermediate the end members of the stack arearranged and constructed such that the insulating rods inserted intoholes on opposite sides thereof have sufficient depth to overlap by aprescribed amount so as to compensate for thermally induced expansion ofthe stack.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectionalview taken across the diameter of a laser tube structure constructed inaccordance with the present invention;

FIG. 2 is a partial perspective view of the structure shown in FIG. I;and

FIGS. 3a and 3b are drawings of a simplified rigid body structure whichare useful for explaining the operation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, precisionbore quartz insulator tube 10 is sealed at each end by quartz Brewsterwindows 11 and 12 and interconnected with quartz ballast tank 13 to forman enclosed container, commonly referred to as a vacuum envelope, inwhich argon is confined at a pressure of about onehalf Torr. Neon,krypton and xenon may also be used to function as the ionized gaseouslasing medium but argon is preferred because of its greater efficiencyand higher power output capability. Both the insulator tube and theballast tank are cylindrically shaped and have a wall thickness of about1 millimeter with interior diameters of approximately 1 /2 inches and 3inches, respectively. The ballast tank is generally constructed with avolume equal to or greater than that of the insulator tube since it isused for minimizing pressure variations that are likely to occur duringthe warmup period following the application of electrical power to thelasing medium. Electrically conductive graphite discs 14 are arranged ina stack between notches 16 and 17 in the insulator tube and separatedfrom one another by quartz spacer rods 18. The maximum diameter of thegraphite discs as observed along axis 19 of the tube is slightly lessthan the interior diameter of the tube for easy insertion therein.Axially oriented apertures 21 passing through the center of therespective discs in collinear alignment with one another form asegmented laser discharge bore. The central apertures 21,. in graphitediscs 14,. at each end of the stack expand conically from a diametersubstantially equal to that of the discharge bore at the ends remotefrom the adjacent Brewster windows to a considerably larger diameter atthe opposite end to provide a smooth transition for the laser beambetween the small area of the segmented discharge bore and the largerarea of the insulator tube. This construction minimizes ion sputteringdamage to the ends of the discharge bore.

To achieve lasing action in a discharge bore made of a conductivematerial such as graphite, the length to diameter ratio of the centralaperture must be within prescribed limits. In general, the ratio shouldbe less than to l and preferably about 5 to 1. Thus, for a dischargebore diameter of 2 millimeters, graphite discs 1 centimeter in lengthforming a stack about one-half meter long have operated satisfactorilyin a tube measuring 1 meter between the Brewster windows.

Passages 22 displaced from the center of the graphite discs function asgas return lines to maintain equalized pressure throughout the length ofthe segmented discharge bore. In the absence of these gas return lines,ions flowing towards one of the electrodes in a DC excited laser wouldtend to increase the pressure at one end of the tube and decrease it atthe other end resulting in diminution or perhaps even cessation oflasing action. To assure that lasing action cannot be established inthese passages, their length to diameter ratio must be greater than thecorresponding ratio for the discharge bore. Thus, since the length ofthe gas return lines is equal to that of the discharge bore, theirdiameter must be smaller. To assure that the gas pressure is equalizedthrough the length of the segmented discharge tube, several gas returnlines are generally used as indicated in FIG. 2.

The excitation for stimulating the laser discharge is applied to the gasby coupling the positive and negative terminals respectively of a DCsource (not shown) to leads 23 connected to anode terminals 24 and toleads 26 connected to cathode 27. To minimize noise, the cathode ispositioned symmetrically and in close proximity to the discharge bore inthe graphite discs. A barium impregnated thyratron type cathode is usedto provide the required high emission current densities for the life ofthe laser. Anode 28 is formed by a graphite disc having a centralaperture 29 which tapers from a large diameter at the end proximate thegraphite stack to a diameter which is substantially smaller but stilllarger than the discharge bore diameter. This construction limits thediameter of the discharge beam and precludes the possibility of theanode leads being scared by the laser beam.

As previously mentioned, since graphite is a refractory material havingrelatively high thermal conductivity compared to quartz, it is able towithstand the high operating temperature of the laser discharge and toquickly radiate the heat therefrom. Moreover, the vacuum envelope issuitably formed of quartz in this instance, since the gas discharge isnot directly in contact with it and its interior wall area isconsiderably larger than in the case where it is used to form thedischarge bore thereby reducing the heat applied to it. In addition, ionsputtering of the envelope is substantially reduced thereby decreasingthe likelihood of catastrophic puncturing. Graphite, however, ischaracterized by a comparatively high thermal coefficient of expansionin the direction parallel to the axis of the discharge bore.Consequently, when the laser is in operation, the extreme heat in thedischarge bore causes the stack to expand longitudinally and force againnotches I6 and 17 thereby placing the quartz tube under considerabletension and increasing the likelihood of cracking or shattering of thetube. Radial expansion of the graphite is rather small so there is noconcern about the discs forcing against the sidewall of the insulatortube.

In the present invention, the longitudinal expansion of the graphitestack is compensated by quartz spacers 31 inserted in spacer holes 32 inthe discs. Referring to FIG. 2, it is seen that the spacer holesextending into each disc from opposite sides thereof are not collinearlyaligned. This construction enables the spacer holes to extend more thanhalfway through the discs. The depth of the holes in the end discs isnot critical. In the discs intermediate the end discs, though, the depthof the holes extending into each disc from opposite sides thereof isclosely controlled to provide a predetermined amount of overlap d. Morespecifically, to achieve precise temperature compensation, the totaloverlap obtained by adding the overlap in all of the intermediate discsis made equal to the sum of the distances D between the bottom of thespacer holes and the opposite surface of each of the end discs. Thus,for a stack comprising a total of 12 discs, two end discs plus 10intermediate discs, 2D=10d.

The manner in which this construction provides thermal expansioncompensation will be more clearly understood by referring to FIGS. 3aand 3b. In FIG. 3a, assume that rods 36, 37 and 38 are rigidly coupledtogether and connected between fixed surfaces 39 and 41. As rods 36 and38 expand in the directions indicated by the arrows to move to thepositions shown by dashed lines 42 and 43, rod 37 will be placed undertension unless it also expands to a length equal to the distance betweendashed lines 42 and 43. The same conditions prevail in FIG. 3b where thegraphite discs 44 are positioned between fixed surfaces 46 and 47 andrigidly coupled by rods 48,,, 48,, and 48 extending to the bottom ofspacer holes 49 49,, 49,,, 49, 49, and 49,. The quartz spacers 31 usedin FIG. 1 are rigid members having a longitudinal thermal coefiicient ofexpansion an order of magnitude less than that of the graphite discs andtherefore may be regarded as providing a rigid, temperature insensitivecoupling between the discs in the same manner as rods 48,,, 48,, and 48When the bottom of the spacer hole 49,, moves to the position indicatedby dashed line 51 as the temperature rises, the bottom of spacer hole49,, will be constrained to move to the position indicated by dashedline 52. Likewise, the bottom of spacer holes 49, and 49, will move tothe positions indicated by dashed lines 53 and 54 when the temperaturerises. If the length of the overlap region d extends to a length equalto the distance d between dashed lines 52 and 54, the center disc willnot be placed under tension. The same situation applies to spacer holes49,, and 49,, relative to holes 49 and 49, Thus, by providing an overlapregion having a length equal to the sum of the distances D and D thestack is compensated for thermally induced longitudinal expansion.

In the absence of temperature compensation provided in accordance withthe foregoing description, it should now be apparent that each of theintermediate discs would be exerted on by compressive forces as thetemperature rose. The inability of the discs to compress, however, wouldcause the entire stack to expand longitudinally against the end notchesin the insulator tube and thereby place the tube under tension as statedpreviously.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

We claim:

1. In a laser apparatus comprising an insulator tube enclosing alongitudinally disposed array of refractory members held spatiallyseparated from one another by insulating members and each having anaperture extending parallel to the longitudinal axis of the insulatortube such that the respective aper tures align to form a discharge path,the improvement wherein the refractory members have spacer holesextending partially therethrough in a direction parallel to butdisplaced from the discharge path forming aperture, .the spacer holes onone side of the refractory members intermediate the end refractorymembers being misaligned from the spacer holes on the opposite side ofeach of said intermediate refractory members thereby enabling saidspacer holes to have sufficient depth to overlap by a prescribed amountin proportion to the distance between the bottom of the spacer holes inthe end members and the side of the end members remote from theintermediate members, and

the insulating members are rods positioned in the respective spacerholes such that each rod extends from the bottom of a spacer hole in onerefractory member to the bottom of a collinearly aligned spacer hole inan adjacent refractory member and thereby operates to maintain alignmentof the discharge beam forming aperture.

2. The apparatus of claim 1 wherein the individual spacer holes in agiven side of each refractory member have substantially the same depthand the aggregate overlap of a pair of spacer holes in each of theintermediate refractory members is substantially equal to the sum of thedistances between the bottom of a spacer hole in each end refractorymember and the opposite side thereof.

3. The apparatus of claim 1 wherein the aggregate overlap of the spacerholes in the totality of the intermediate refractory members issubstantially equal to the sum of the distances between the bottom of aspacer hole in both end refractory members and the respective oppositesides thereof.

4. The apparatus of claim 3 wherein the active lasing medium containedwithin the insulator tube is a gas selected from the group consisting ofargon, xenon, krypton and neon.

5. The apparatus of claim 3 wherein the refractory members are graphitediscs in which the discharge path forming aper ture is centrally locatedand the insulating members are fused quartz rods.

6. The apparatus of claim 5 wherein the central aperture in the endgraphite discs is conically flared from a diameter approximately equalto the diameter of the aperture in the intermediate discs at the endadjacent thereto to a substantially larger diameter at the opposite endand the graphite discs further include a bypass channel displaced fromand having a smaller diameter than the diameter of the central aperture.

2. The apparatus of claim 1 wherein the individual spacer holes in agiven side of each refractory member have substantially the same depthand the aggregate overlap of a pair of spacer holes in each of theintermediate refractory members is substantially equal to the sum of thedistances between the bottom of a spacer hole in each end refractorymember and the opposite side thereof.
 3. The apparatus of claim 1wherein the aggregate overlap of the spacer holes in the totality of theintermediate refractory members is substantially equal to the sum of thedistances between the bottom of a spacer hole in both end refractorymembers and the respective opposite sides thereof.
 4. The apparatus ofclaim 3 wherein the active lasing medium contained withIn the insulatortube is a gas selected from the group consisting of argon, xenon,krypton and neon.
 5. The apparatus of claim 3 wherein the refractorymembers are graphite discs in which the discharge path forming apertureis centrally located and the insulating members are fused quartz rods.6. The apparatus of claim 5 wherein the central aperture in the endgraphite discs is conically flared from a diameter approximately equalto the diameter of the aperture in the intermediate discs at the endadjacent thereto to a substantially larger diameter at the opposite endand the graphite discs further include a bypass channel displaced fromand having a smaller diameter than the diameter of the central aperture.